1. Field of the Invention
This invention relates in general to non-contact polishing of semiconductor materials and specifically to the use of polishing solutions.
2. Description of the Prior Art
Polishing of semiconductor wafers is an important step in the fabrication of many semiconductor device structures. For example, flat surfaces are needed for the growth of epitaxial layers. Current methods of cutting and polishing semiconductor wafers can produce a region of extensive damage underneath the surface of mechanically polished wafers. Despite attempts to remove this subsurface damaged region by chemical polishing, residual damage is found in most commercially available polished substrates. The depth of the damaged region varies from a few to several tens of micrometers.
The presence of such a damaged region can seriously affect the performance and reliability of devices fabricated on these substrates. As a result, a need exists to develop polishing methods for removing the damaged region without inflicting further mechanical damage on the substrate surface.
Traditional methods of polishing semiconductors involve one or more of the following contact techniques. The most commonly used technique is abrasive polishing. In this technique, the semiconductor wafer is lapped in a slurry containing a small diameter abrasive (&gt;5 .mu.m), which on contact removes material through mechanical abrasion from the surface that is being polished. Finer and finer abrasives are used until the desired surface finish is achieved. Limitations of this technique are that the ultimate size of the abrasive in the slurries dictates the quality of the final finish and that the uniaxial pressure applied to the sample can create subsurface damage that will remain after the polishing process.
Chemical polishing is commonly used to remove damage left behind by mechanical polishing. In chemical polishing, material removal is achieved by the chemical reaction between the substrate and the polishing solution. It typically requires that the substrate be mounted on a holder and then lapped with a cloth saturated with the polishing solution. Although it produces a highly polished surface, this technique tends to generate a surface damaged region caused by mechanical stress resulting from physical contact between the substrate and the polishing surface. For structurally weak materials such as compound semiconductors, damaged regions extending as far as 40 .mu.m into the bulk have been reported.
Other methods involve a variation or combination of these techniques. Consequently, compound semiconductor substrates polished by these contact methods tend to retain a highly damaged region beneath the polished surface.
Recently several non-contact methods have been proposed. These methods claim to produce no subsurface damage. One such method called "Magna-Smooth" is an adaptation of the bowl-feed process developed by VTI Inc. of Dayton, Ohio. The bowl-feed method polishes the semiconductor surface by having the sample immersed in a polishing slurry. No fresh abrasives are added during polishing. As the process continues, abrasives are pumped up to the sample and ground finer and finer as they are reused. Gradually, the abrasives are allowed out to settle so that in the final polishing stages, the slurry becomes almost pure water. The polished surface is said to have an RMS roughness and waviness in the 1 to 5 angstrom range. Liabilities of this method are that the process generates enormous pressures on the material, especially in its final, near pure water polishing phase. These pressures are high enough to cause the polished surface to flow. Close examination of the polished surface indicated the formation of a Beilby layer which exhibits morphology different from that of the bulk.
A current state-of-the-art non-contact method is disclosed in U.S. Pat. No. 4,323,422, entitled "Method for Preparing Optically Flat Damage-free Surfaces.+ The method involves a non-contact polishing technique called hydroplane polishing. In this method, the substrate is allowed to hydroplane on a layer of polishing fluid between the substrate and a platen rotating at high velocity.
In this technique, conditions are adjusted such that the substrate surface is allowed to hydroplane on a layer of polishing solution between the substrate and a rotating platen (column 1, lines 48 to 51). Using this method, about 50 microns of material may be removed before the position of the substrate requires readjustment (column 3, lines 50 to 55). The technique is claimed to be capable of producing surfaces which are optically flat within 3000 angstrom over about 80% of the sample.
Successful hydroplaning depends on the viscosity, flow viscosity and surface tension of the polishing solution. The surface tension of the fluid, which increases with its viscosity, exerts a force pulling the substrate toward the polishing pad, counteracting the hydroplaning force. Claimed to be effective in leaving the polished surface undamaged, the technique is difficult to control in practice because a set of highly complex hydrodynamic conditions must be maintained during the polishing process. For example, in high surface tension liquids, this force can be large enough to cause the substrate and polishing pad to come in contact with each other. When this occurs, the substrate can be severely damaged (column 3, lines 10 to 23).
It is therefore a principal object of the invention to provide a non-contact polishing method that is fast, inexpensive, and easy to control.
It is an object of this invention to provide a polishing technique which does not suject the surface to no mechanical stress.
It is also an object of this invention to provide a polishing apparatus that is inexpensive to construct and simple to maintain.
It is a further object of this invention to provide a polishing process that is suitable for automation.